The present invention relates to jet turbine engines and, in particular, relates to the static shrouds around the turbine blades and the blade-tip outer air seals.
Efforts to plasma spray thick abradable thermal barrier coating (TBC) systems to ceramic matrix composites (CMC) being developed for turbine shrouds (blade-tip outer air seals) has been inhibited by poor bonding to the silicon nitride substrate. Oxidation resistant CVD silicon nitride bon coatings, which were evaluated to enhance the oxidation resistance of cut SC fibers and the reaction bonded silicon nitride matrix, cracked extensively and locally spalled. Erratic thermal spray deposition of tantalum oxide and mullite coating was achieved with the use of grit blasted uncoated CMC and microcracked CVD SiC bond coated CMC substrates. However, successful dposition was only achieved when the CMC was preheated to above approximately 1000 degrees Centigrade. Severe oxidation of the CVD SiC bond coated CMC was observed during a 90 hour pre-coating furnace oxidation heat treatment at 1200 degrees Centigrade in air.
Thus, there exists a need for a better abradable TBC for CMCs.
A bond coating is applied between a ceramic matrix composite and a insulative and abradable coating layer.
The bond coating is a slurry layer of fine particles of silicon nitride and silicon carbide suspended in a colloidal silica or zircon binder. The ratio of these materials in the bond coating should match the ratio of these in the CMC. If the CMC is SiC fibers in a SiC matrix, the slurry should be composed of SiC particles with a colloidal silica binder. High surface roughness is achieved by entrapping coarse ceramic particles into the wet bond coating slurry. Thereafter the product is heat treated to maximize bond strength and verify oxidation resistance.
Therefore, one object of the present invention is to provide an abradable thermal barrier coating (TBC) for turbine shrouds.
Another object of the present invention is to provide a TBC for using on silicon nitride substrates.
Another object of the present invention is to provide a TBC that are oxidation resistant.
Another-object of the present invention is to provide a TBC for the coating of ceramic matrix composites.
Another object of the present invention is to provide a TBC having good thermal expansion and oxidation resistance.
These and many other objects and advantages of the present invention will be ready apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings.